U.S. Pat. No. 4,961,863 describes an FCC reactor vessel comprising at its upper end cyclone means to separate catalyst particles from an effluent of a dilute phase fluidized catalytic cracking reactor riser, which cyclone means are fluidly connected to the downstream part of the reactor riser, fluidly connected with means to discharge the cleaned reactor riser effluent from the vessel and fluidly connected to dipleg means to discharge the separated catalyst to the lower end of the vessel. A secondary gas discharge opening will be present, generally between or after the separation means, to allow gasses present in the reactor vessel to be discharged from the reactor vessel together with the cleaned reactor riser effluent. Such configurations wherein these means are fluidly connected, resulting in short contact times and low after cracking, are referred to as so-called closed-cyclone or close-coupled FCC reactor configurations.
In use coke tends to form on, for example, the surface of the separator means of an FCC reactor vessel of for example U.S. Pat. No. 4,961,863. This is especially the case when an FCC unit is operating on a more heavier feedstock than it was designed for. This is because small amounts of coke precursors, for example heavy hydrocarbons, are not fully separated from the catalysts in the separation means. When these small amounts of coke precursors together with the catalyst and are discharged to the lower end of the reactor vessel they will almost immediately separate from the catalyst and flow upwardly in the reactor vessel. Coke will form when these coke precursors come into contact with the hot exterior of for example the separator means.
The problem of excessive coke formation is especially a problem when processing heavy feedstocks in FCC reactor vessels which have been modified from a conventional non-close coupled design into the so-called close-coupled design. If the coke formation reaches a certain level large fragments of coke can to drop down into the lower end of the reactor vessel. These large coke fragments can in turn cause blockage of the means to discharge catalysts from the reactor vessel. Due to such blockage the FCC unit will have to be shut down in order to remove the blockage. It has been experienced that such unscheduled shutdowns occurred already after 1 to 2 years of operations. This is very disadvantageous. Especially considering the fact that an FCC unit is supposed to operate without an unscheduled shut down for many years, for example 4 years.